Power gained from valves with slimmed stems??

Josh,

In my limited experience, there appears to be no power gain that is directly associated with slimming of the valve stem. The main benefit comes from weight reduction which can allow for higher RPMs. This comes at the price of valve durability and a reduction in the valve's ability to conduct heat.

Regards,
Bryan Carter

I don't often dissagree with Bryan, but in this case, I have to at least qualify his statements

The valve stem will in fact be strengthened re breaking strength if the stem dia is turned down to match the root dia of the retainer groves, however the valve will bend easier, and stretch easier. It will also as Bryan indicates, loose some of it's ability to conduct heat from the head, down the stem to the guides.

Some engines give more power with slim valve stems, it all depends on where the restrictions are in the port and streamlineing of the guides etc. All in all, it is easier to get air to flow around a thin stem than a thick stem.

Increasing valve head size is also maybee, maybee not, depending on the original size and bore size and chamber shape. Generally bigger is better, but the bigger the valve, the more critical it becomes to have a chamber design that does not shroud the opened valve. eg if a valve almost touches the cylinder bore, not much gas can flow around that side of the valve.



Regards
pat

Darn it Pat! I thought we'd agreed that you wouldn't disagree with me anymore!

Seriously, let me make myself a little clearer. The theory behind turning valve stems down is that it will help with airflow around the stem. Obviously a thinner stem will present less of an obstruction to airflow than a thick one.

Different engines with varying valve include angles and port configurations will see varying degrees of improvement after performing this modification. I know many people speak highly of this trick, but I have only experienced with it briefly. On the particular applications that I tried it with (little motors, bug turbos, high boost) I saw little return (in terms of HP) on my investment, and picked up a severe exhaust valve cooling problem for my trouble.

So, for me, it's one of those mods I can take or leave. By the way, the sodium that's inside of sodium-filled exhaust valves... burns like the devil if you get it on your hands. Don't even ask me how I know this.

Good Luck,
Bryan Carter

My experiences are mainly with Small Block Chevs, and they seem to give a small improvement there, wether or not it's value for money is a much more subjective judgement.

I guess it's not a good idea to cut opened a sodium filled valve, as sodium plus water equals caustic soda plus heat.

Skin contains water

Ouch s**t d*m f**k, how do I wash this off, ouch.



Regards
pat

One trick in the V8 world is to turn down the stem only in the area between the hard chrome which travels inside the guide and the head of the valve. The valve remains at the full .341" Ø in the guide and cuts down to .310 in the throat area. (street applications)

Of course V8 race engines are using smaller diameter valves for weight reduction. I believe they are around 7mm (Ø.276)

Hopefully you're just thinning the intakes and not the exhausts. The last thing an exh. valve needs is a reduced dia. right where the stem is hottest and therefore weakest. The noise an engine makes when a valve head breaks off is a symphony written in $ major!

Actually, I am having both made with slimmer stems. The stock stems are 7mm, I am having them made with 6mm on both sides.

My supplier/manufacturer said there will be no problems whatsoever with this. They have applications for Honda engines with smaller 5mm stems and much higher RPMS with no problems.

The only way to test what I am about to say is on a flow bench. On many heads, great gains in exhaust flow can be had through the use of a tulip head valve rather than a nail head type. When the flow picks up, it lessens exhaust cam duration requirements thus keeping more torque in the motor. Tulips don't work in everything, and port shape makes or breaks the case, but given a fair shot on the flow bench, I am sure they will give you something. Stock small and big block chevy heads sure like them.

Happy flowing.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

Just because it survives on honda's does not mean it will survive the VW, honda is known for building higher rpm motors, this mean that their valves are designed lights, and thus the stem can be smaller, personally I am sure there might be some power gain from this, however you will gain more by increasing the valve size and port by 1mm...